Slashing carbs to this degree forces the body into ketosis, a metabolic state in which your body starts to rapidly break down fat for fuel. That sounds better than it feels. During the first few days—sometimes weeks—of ketosis, people often feel tired, irritable, and dizzy and have difficulty sleeping. This carbohydrate withdrawal is often called the “keto flu,” explains registered nutritionist and certified dietitian Dana Notte, MS, director of nutrition at Green Mountain at Fox Run, a healthy-weight retreat known for helping women “detox” from fad dieting. Check out the 13 things everyone gets wrong about the keto diet.

Shrinking muscle

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Contrary to what most people think, the keto diet isn’t a high-protein plan; it’s really about cutting carbohydrates and protein. And that protein deficit can be hard on your muscles, research suggests. When you limit carbs from things like fruits, vegetables, and grains, your body starts to steal protein from your muscles and other tissues to transform it into carbohydrate, Notte says. “Protein makes up our organs, our heart, our muscles as well as some hormones, antibodies, and neurotransmitters.”

One of the reasons people see rapid weight loss at the start of a ketogenic diet, Notte explains, is that you’re losing actual muscle, not just fat. This info is often a mind-blower for Notte’s clients at Green Mountain, many of whom have spent years losing and regaining pounds, she says. Find out exactly why most people fail on keto.

Brain fog

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When ketosis kicks in, the body switches from stealing protein for fuel to burning fat. “The body starts to rely more heavily on fat stores,” Notte explains. Molecules called ketones are released when body fat is metabolized, and some cells in the brain can use ketones for fuel. However, your brain needs more than 100 grams of carbohydrates a day, according to research, and while your body makes the switch to relying on fat—ketones—for energy, your brain can suffer.

“Brain fog” is a common complaint among people on extremely low-carb diets like keto. “I remember one client in particular who had been put on a keto diet for her PCOS [polycystic ovary syndrome] who kept telling me how much better she felt after eating in a more balanced, more nourishing way for a couple of weeks at our program,” Notte says. Make sure you know these 15 things before starting the keto diet.

Dehydration and leg cramps

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Another reason for the quick weight loss when you start a keto plan is water loss. “Nitrogen is one by-product of breaking down protein for fuel,” says Notte. “Free nitrogen floating around the body is toxic, so the body flushes it out with frequent urination.” This loss of fluids shows up as pounds lost on the scale. But it can also lead to dehydration and an imbalance of electrolytes—the minerals and salts your muscles rely upon. This is why some keto dieters get terrible leg cramps, Notte explains. “The same thing happens with the Atkins diet—keto is just today’s version.” Make sure you know what happens to your body on keto.

More stress

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A keto diet is very restrictive—more so, even, than plans such as paleo and Atkins. One piece of fruit or a serving of steamed beets can put someone over his or her entire daily carbohydrate limit. “Restrictive dieting is a psychological stressor,” says psychotherapist Shiri Macri, MA, LCMHC, clinical director at Green Mountain at Fox Run, who specializes in working with people who emotionally overeat or binge-eat. “When people deprive their bodies of the amount of nutrition and the nutrients that it needs, the body responds in a state of emergency. Levels of cortisol, a stress hormone, go up, and we also have a decrease in mood-boosting serotonin.” Check out the 11 hidden dangers of the keto diet.

Rebound eating

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This is a well-known phenomenon with nearly every type of diet: If you relax the rules even a little bit, you suddenly find yourself overeating. Making entire food groups—such as grains, sugar, and other carbohydrates—off-limits helps make them even more attractive, Macri says. This type of reaction goes beyond a psychological reaction—it can also be physical, she says. “Serotonin is one of the feel-good brain chemicals, and when serotonin decreases, such as it does when you’re on a strict low-carb diet, the brain is on high alert for any kind of reward,” she explains. Let some sugar or bread into your diet, and you could be overcome with a desire for more, Macri warns. “This makes it much more likely that you’ll overeat or even binge on those foods.”

Weaker bones

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Ketones—the molecules produced during fat metabolism on the keto diet and other very low-carb diets—are acidic. “When they start to build up in our blood, the blood becomes more acidic,” Notte says. One of the ways the body balances high acid levels is by using a base like calcium, she explains. “The body pulls this alkaline mineral from our bones.” In the long term, chronic calcium loss can lead to low bone density, osteoporosis, and fractures. Try these 15 ways to boost bone health.

Constipation

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With grains, most fruits, and many vegetables off the menu, your digestive tract will bog down. “It’s really hard to get adequate fiber eating that way,” says Notte. “A common side effect is significant constipation.” Not only is constipation unpleasant, but it’s also a sign that your gut isn’t happy. “Everything in the keto diet is low in fiber—and that does not support gut health.”

Disordered eating

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“When you’re on a restrictive diet, deprivation builds and builds, and when you finally give in and have something that’s not on your plan, you can overeat,” Notte says. This can lead to shame and guilt, she warns. “You think, well, I already blew it, so I might as well just eat it all,” she says. Occasional binge eating can, in turn, develop into a full-blown disorder called binge eating disorder for those who are predisposed to it, according to the National Eating Disorders Association. Check out the 10 surprising health benefits of keto.

According to Macri, “If somebody is looking for physical and psychological health, the best way to get there is to focus on health. That entails eating in balance, not restrictively; moving regularly; and looking at the role of stress in one’s life. Those three areas will get someone to health as well as happiness.” Check out the 13 things doctors want you to know about the keto diet.